In a facsimile apparatus, for example, analog picture image signals produced by scanning printed matter or a manuscript with a line sensor are binarized (converted into binary codes) according to a predetermined threshold value. The level of the image signal varies variously depending upon such factors as the tone and line size of the picture image, whether the line is vertical or horizontal, and the state of surrounding picture image, etc. If the level were simply binarized, however, it would be impossible to obtain reproduceable binarized picture image signals. For example, since a fine black line in a white background, a continuous pattern of fine white and black lines, and a white fine line in a black background have different signal levels, it has been impossible to obtain all of the information for these lines regardless of the settings of their threshold values.
For this reason, in the past, a correction circuit was provided before a binarizing circuit for correcting signal levels of picture image signals. However, as will be described later in detail with reference to the accompanying drawings, the prior art correction circuit can not efficiently correct specific image signals.